In the prior art, various methods and apparatus have been employed to mix liquids and slurries, such as paints containing pigments which must be uniformly dispersed within the liquid base in which they are contained. In the case of paint mixing in particular, the apparatus utilized has ranged from small impellers employed as motor driven stirrers for laboratory uses to large, heavy duty, motor driven propeller type impellers used for mixing paint in 55 gallon drums and larger sizes.
In the case of the heavy duty apparatus in particular, such apparatus has typically comprised a long, flexible shaft with a propeller shaped impeller mounted at one end of the shaft and a motor drive attached at the opposite end. In some cases, where the liquids involved are particularly viscous, additional propeller type impellers have been mounted along the length of the shaft to provide additional mixing concentration points within the volume of the liquid.
In the typical heavy duty mixing apparatus of the type just described, a mixing vortex flow is created within the liquid in which liquid flow is pulled downwardly in a swirling pattern in the center of the container and then ascends back to the top of the container along the outer peripheral edges thereof. This type of descending swirling flow pattern causes the entrapment of air in the liquid being mixed, which creates a number of problems. Because the container is typically open at the top thereof during the mixing operation, the air escapes from the liquid after entrapment and produces a high concentration of fumes or, to the extent it remains entrapped for a period of time, interferes with the operation of on-site equipment, such as spray painting equipment and other dispensing equipment connected to dispense paint on-site directly from the container after mixing.
Because of environmental concerns, government regulations have increasingly required that such heavy duty paint mixing operations be carried out using covered containers. However, since access to the liquid being mixed is typically required repeatedly by the operator during the mixing operation to obtain samples and the like, operators have tended to avoid using covers provided for this purpose during mixing operations, because the covers must be repeatedly removed during the operation to gain the required access.
In addition, the above-described prior art apparatus and methods have a tendency to leave substantial amounts of unmixed pigments on the bottoms of the drums. The fact that some substantial portions of the pigments remain unmixed means that the resulting emulsion does not contain the concentration of pigment which is required, the result being a product not only of poorer quality but also one in which the color is not properly matched to the specification.
One of the objects of the present invention is to provide an improved mixing apparatus and method which overcomes the aforementioned disadvantages and which provides, in addition, a highly efficient and rapid mixing operation which has particular advantages for on-site mixing applications. Other objects and advantages of the present invention will be set forth below and will become apparent from the description herein set forth.